In recent years, new types of laser light sources using a photonic crystal have been developed. A photonic crystal consists of a dielectric body material in which an artificial periodic structure is created. Usually, the periodic structure is created by providing the body material with a periodic arrangement of areas whose refractive index differs from that of the body material (this area is hereinafter called the “modified refractive index area”). The periodic structure causes a Bragg diffraction within the crystal and creates an energy band gap for the energy of light. There are two types of photonic crystal laser light sources: one utilizes a band-gap effect to make a point-like defect function as a resonator, and the other utilizes a standing wave at a band edge where the group velocity of light becomes zero. Each of these devices causes a laser oscillation by amplifying light of a predetermined wavelength.
Patent Document 1 discloses a laser light source in which a two-dimensional photonic crystal is created in the vicinity of an active layer containing a luminescent material. The two-dimensional photonic crystal includes a plate-shaped semiconductor body in which cylindrical holes are periodically arranged (e.g. in a triangular, square or hexagonal lattice pattern) so as to provide the crystal with a two-dimensional, periodic distribution of refractive index. Its period is adjusted so that it equals the wavelength of light to be generated within the active layer by an injection of carriers from an electrode. As a result, a two-dimensional standing wave is produced within the two-dimensional photonic crystal, whereby the light is strengthened to produce a laser oscillation. The laser light thereby produced is diffracted by the two-dimensional photonic crystal to a direction perpendicular to the same crystal. Thus, a two-dimensional emission of light from the surface of the two-dimensional photonic crystal is obtained.
FIG. 1 shows a calculated result of the electromagnetic distribution of a standing wave created within the two-dimensional photonic crystal with holes 11 arranged in a square lattice pattern in the laser light source disclosed in Patent Document 1. Thick circles 11 in FIG. 1 each indicate the hole 11, the gray shading represents the magnetic field strength, and the direction and length of the arrows respectively indicate the direction and magnitude of the electric field. The electric field encircles the (gravity) center of each hole 11 and is anti-symmetrical with respect to the gravity center. If the two-dimensional photonic crystal has an infinite extent, the anti-symmetry of the electric field will result in the cancellation of the electric field at every hole due to interference (destructive interference), so that the laser light will not be emitted to the outside.
Actually, the extent of the two-dimensional photonic crystal is finite. Therefore, the anti-symmetry is broken and the electric field cannot be completely cancelled, so that a portion of the laser light generated within the two-dimensional photonic crystal will be emitted to the outside. However, the laser light to be emitted to the outside is weakened due to the influence of the interference of the electric field.
Patent Document 2 discloses a surface emitting laser light source having a two-dimensional photonic crystal in which the lattice structure, which is decided by the combination of the lattice pattern and the shape of the modified refractive index area, is translationally but not rotationally symmetrical. One example of such a two-dimensional photonic crystal is the one in which the rotational symmetry of the lattice differs from that of the modified refractive index areas, as in the case where the modified refractive index areas are arranged in a square lattice pattern and their shape is an equilateral triangle. The two-dimensional photonic crystal in this surface emitting laser light source is structurally less symmetrical than the one disclosed in Patent Document 1, so that the destructive interference is less influential. As a result, the light emitted perpendicularly to the crystal surface becomes stronger.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-332351 (Paragraphs [0037] to [0056]; FIG. 1)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-296538 (Paragraphs [0026] to [0037] and [0046]; FIGS. 1 to 5 and 22)